Thermostructural composite materials are characterized by their mechanical properties that make them suitable for constituting structural elements and by their ability to conserve their mechanical properties at high temperatures. Typical thermostructural composite materials are carbon-carbon (C-C) composites and ceramic matrix composite (CMC) materials.
C-C composites are constituted by a reinforcing texture or "preform" of carbon fibers that is densified by a matrix of carbon. CMCs are constituted by a preform of refractory fibers (carbon fibers or ceramic fibers) densified by a ceramic matrix. A ceramic material commonly used for manufacturing CMCs is silicon carbide (SIC).
The preform of a C-C composite or of a CMC is made by stacking or draping unidirectional plies (sheets of mutually parallel yarns or cables) or multidirectional plies (pieces of woven cloth, webs of fibers, pieces of felt), or by winding yarns, tapes, or strips, or by three-dimensional weaving. When draping plies, they may be bonded together by needling, by sewing, or by implanting transverse threads. Preforms are made of ceramic or carbon fibers, or more generally of fibers made of a precursor of ceramic or carbon, with the precursor then being transformed after the textile operations required for manufacturing the preform have been completed.
The purpose of densifying a preform is to fill the accessible pores thereof with the matrix-forming material. Such densification can be implemented by impregnating the preform with a liquid that contains a precursor of the matrix material and then transforming the precursor, or by chemical vapor infiltration.
The techniques mentioned above for making fiber preforms out of carbon or ceramic, and for densifying them by means of a carbon matrix or a ceramic matrix are well known.
Several methods exist for manufacturing honeycomb structures.
A first known method (FIGS. 1A, 1B, and 1C) consists in stacking together sheets 10 and in gluing them together in a staggered configuration. Gluing takes place along parallel strips 12 with strips of glue situated on one face of a sheet being offset relative to strips of glue situated on its other face (FIG. 1A). The set of sheets is then cut up into slices 14 perpendicularly to the strips of glue. Each slice is then stretched in the direction normal to the faces of the sheets (arrows f in FIG. 1B) so as to obtain hexagonal cells 16 by deformation (FIG. 1C). A honeycomb panel 18 is then obtained, and metal or composite sheets may be stuck on its opposite faces.
Such a method is used for making metal honeycomb structures. The sheets 10 are cut out from sheet metal, and the cells 16 are produced by plastic deformation of the metal.
That method can also be implemented using sheets of card or of paper. In which case, after being stacked and glued in a staggered configuration, the sheets of paper may be impregnated with a resin, e.g. a phenolic resin. The resin is cured after the cells have been formed (which happens either before or after the set of sheets has been cut up into slices).
To make a honeycomb structure out of a thermostructural composite material, it would be possible to implement a method of the same type using two-dimensional fiber plies, e.g. plies of woven cloth that are stacked and glued in a staggered configuration. Densification and consequently rigidification of the structure would then be performed after stretching and cell formation. Each ply would normally be made up of a plurality of layers of cloth, thus requiring the layers in a given ply to be bonded together in order to prevent them separating during stretching. In addition, it is difficult to perform a gluing operation in a staggered configuration on cloth with the regularity and accuracy required for ensuring that cloth is not torn because of a local defect when stretching is applied. In addition, during the densification operation after stretching, there is a danger of thermal stresses rupturing the glue.
One solution would be to sew the plies of cloth together in a staggered configuration, instead of gluing them together, but although that would avoid certain drawbacks, it would also give rise to considerable difficulties of implementation.
A second known method (FIGS. 2A, 2B) consists in using corrugated sheets, e.g. of metal foil. The corrugated sheets 20 are superposed and glued or welded or soldered together along their touching facets 22 (FIG. 2A). Honeycomb panels 28 are obtained directly by slicing the block of sheets 20 perpendicularly to the corrugations (FIG. 2B).
That method can be used for making honeycomb structures out of composite material by using corrugated sheets that are themselves made of composite material. Such sheets can be obtained by draping layers of cloth so as to give them the desired corrugated shape and then densifying them, e.g. by draping and molding layers of cloth that have been preimpregnated with a resin or with some other liquid precursor for the matrix of the composite material, and then applying heat treatment. A method of that type is described in document WO 91/16277. However, it is then necessary to glue the corrugated sheets together in a manner that is effective and capable of withstanding the operating temperatures to which thermostructural materials may be subjected in use. In addition, the operations of prefabricating corrugated sheets are lengthy and expensive, thereby considerably increasing the cost of the honeycomb structure.
Finally, a third known method (FIGS. 3A and 3B) uses a sheet 30, e.g. a metal sheet, in which cuts 32 are formed. The cuts are formed in a staggered configuration along parallel lines (FIG. 3A). The cuts are of equal length and they are regularly spaced apart along each line. The cuts situated along one line are offset relative to those in the adjacent lines, and each cut extends over a length that is greater than the distance between two adjacent cuts in the same line. The sheet 30 is expanded by opening the cuts and forcing metal out of the plane of the sheet so as to form cells 36 at the locations of the cuts by plastic deformation of the metal (FIG. 3B). The expansion is limited so as to avoid generating stresses that could tear the sheet, particularly at the ends of the cuts 32. The axis of each cell is inclined relative to the initial plane of the sheet through an angle of less than 90.degree. such that the walls of the cells are not perpendicular to the general plane of the resulting honeycomb panel 38.
The expanded metal technique is practically impossible to transpose to composite materials. They do not have the same capacity as metal for plastic deformation. Expanding layers of cloth prior to densification and rigidification runs a high risk of tearing the cloth at the ends of the cuts, and gives rise to a problem of holding the expanded cloth in shape. In addition, that method suffers from a major limitation as to the thickness of the honeycomb panel that can be obtained. This thickness is determined by the distance between the lines of cuts, and it must be sufficiently small to ensure that expansion can be achieved fairly easily.